Interactions between motor networks for different behaviors occur frequently in all vertebrates including humans. Even though such interactions are fundamental to vertebrate motor systems, little is known about how they occur at a cellular level. This proposal outlines experiments designed to fill this gap in our knowledge. The work uses a goldfish model system that has interactions between a reticulospinal pathway and a spinal network for rhythmic movement that are very similar to those in mammals, but in a system in that is especially amenable to cellular studies. The work focuses on understanding the role of spinal interneurons and motoneurons in the interactions between the motor circuits. The first aim will characterize the motor output produced during interactions between escape and swimming networks in goldfish, as a foundation for studies of the cellular basis of the interactions. The Mauthner axon, a reticulospinal neuron which initiates the escape, will be fired in the midst of fictive swimming produced by stimulation of the midbrain. This mimics an escape that occurs during swimming. The interactions between escape and swimming will be monitored by extracellular recordings from motor nerves to examine l) whether the escape networks can override and reset swimming networks, 2) whether the interactions depend on the phase of the swimming cycle during which an escape is initiated, and 3) where (in the brain, the spinal cord or both) the interactions take place. Aim 2 will use intracellular recording to study the cellular basis of the interactions in aim l. This aim will determine what spinal motoneurons and interneurons are shared by swimming and escape networks and study events at the single cell level that might explain the motor output produced during interactions between the two networks. Aim 3 will use a combination of extracellular and intracellular recording, and light and electron microscopy, to determine what populations of motoneurons are excited by an identified class of interneuron shared by swimming and escape networks. Aim 4 will use intracellular recording to study the functional characteristics of a pool of interneurons shared by swimming and escape. This aim will determine whether the excitability of an interneuron is related in a systematic way to the strength of its connections to motoneurons. The data from the four aims will provide fundamental, new information about how motor networks in vertebrates interact with one another. It will also address several important questions about the output of spinal interneurons to different motoneuron types and the functional organization of interneuron pools - questions that have not been addressed for any vertebrate. Because the work deals with features of motor organization that are typical of all vertebrate motor systems, including humans, it will have implications well beyond the specific species, pathways, and behaviors studied.